As is well known in the art, in the manufacture of printed electronic circuits a laminate of copper and etch resistant material, usually plastic, is used. A common method of obtaining the circuits is to mask the desired pattern on the copper surface of the laminate with a protective resist material, which is impervious to the action of an etch solution. In a subsequent etching step, the unprotected areas of the copper are etched away, while the masked areas remain intact and provide the desired circuiting supported by the plastic. The resist material can be a plastic material, an ink or a solder.
In the last few years, the industry has more and more turned to hydrogen peroxide-sulfuric acid systems for etching the electronic circuit boards, due to the low cost of the etching solutions and to the relative ease with which copper values can be recovered from the spent etch solutions.
However, there are many problems connected with the use of hydrogen peroxide as an ingredient in the etchants. It is a well known fact that the stability of hydrogen peroxide in a sulfuric acid-hydrogen peroxide solution is detrimentally affected by the presence of heavy metal ions such as copper ions. Thus, as etching proceeds and copper ion content of the etchant thereby increases, the etch rate will experience a serious drop-off due to the decomposition of the hydrogen peroxide in the etch bath, which will soon be exhausted. In order to improve the capacity of these etchants, various stabilizers have been suggested and used with some success for abatement of the hydrogen peroxide decomposition due to the presence of copper ions.
Although considerable retardation of the metal ion induced hydrogen peroxide decomposition can be achieved by the addition of a suitable stabilizer, the etch rates of the stabilized hydrogen peroxide-sulfuric acid etchants have, generally, been quite low and in need of improvement especially at high copper ion concentrations. It has therefore, been suggested in the prior art to add a catalyst or promoter to improve the etch rate. Specific examples of such catalyst are the metal ions disclosed in the U.S. Pat. No. 3,597,290, such as silver, mercury, palladium, gold and platinum ions, which all have a lower oxidation potential than that of copper. Other examples include those of U.S. Pat. No. 3,293,093, i.e. phenacetin, sulfathiazole and silver ion, or the various combinations of any of the above three components with dibasic acids, as disclosed in U.S. Pat. No. 3,341,384, or with the phenyl ureas or benzoic acids of U.S. Pat. No. 3,407,141, or with the urea and thiourea compounds of U.S. Pat. No. 3,668,131.
Another problem often encountered using hydrogen peroxide-sulfuric acid etchants is that etching rates are adversely effected by the presence of even small amounts of chloride or bromide ions, and usually ordinary tap water cannot be used in preparing the etching solution. It is therefore required that these ions be removed either by deionization of the water or by precipitation of the contaminating ions, e.g. when added in the form of a soluble silver salt.
Although silver ions thus appear to provide a universal solution to the above-discussed problem of low etch rates as well as that caused by the presence free chloride and bromide ion content, there are still some disadvantages had with the use of silver ions in preparing hydrogen peroxide-sulfuric acid etch solutions. One of these is the high cost of silver. Another is that silver ions still do not promote the rate of etching as much as would be desired. Still another serious problem sometimes encountered in etching operations for the manufacture of printed circuit boards is the tendency of a formation of scaly solids when employing a metal ion such as silver as a catalyst in the acidified hydrogen peroxide etchant. These solids accumulate around the edges of the boards between or adjacent to the areas coated with the resist. When solder resist techniques are employed, the solder also has a tendency of becoming discolored. The aforementioned deposits and discolorations are undesirable in that they may interfere with the electrical properties of the formed circuit, and if severe enough, can cause short circuiting.
An object of the present invention is therefore to provide a novel, highly efficient aqueous composition for the dissolution of metals without any attendant scale formation or resist discoloration.
Another object is to provide an improved method for clean dissolution of metals, e.g. copper or alloys of copper, at high rates.
Other objects of the invention will become readily apparent from the detailed description set forth hereinafter.